Rates of bloodstream infection in Ireland caused by MRSA remain high, and together with soft tissue and bone/joint infections, continue to pose a serious threat to patients. Exacerbating this problem is the extensive use of implanted biomaterials, which after colonisation by staphylococci cause device-related infections involving biofilms. Because MRSA is resistant to many currently-used antimicrobials including methicillin and related beta-lactam antibiotics, and because these drugs are largely ineffective against biofilms, new treatment strategies are desperately needed. The discovery in 2008 of the cyclic dinucleotide, c-di-AMP, and recent reports of its role in antibiotic resistance, biofilm and virulence indicate that this signalling molecule is fundamentally important, particularly in Gram-positive pathogens. In 2008 we published the first study exploring cyclic dinucleotide (c-di-GMP) signalling in staphylococci, where we uncovered a novel mechanism of biofilm regulation. In 2012, we reported that a mutation in GdpP, a c-di-AMP phosphodiesterase was associated with expression of high-level resistance to methicillin, attenuated virulence and an altered biofilm phenotype in MRSA. Importantly, GdpP is tightly regulated by ppGpp implicating the stringent reponse in c-di-AMP signalling. Data from our laboratory and other research groups indicates that the stringent response is required for high-level methicillin resistance and we hypothesise that therapeutics designed to reduce ppGpp and c-di-AMP levels will reduce methicillin resistance levels offering a novel approach to the management of MRSA wound infections, device-related infections, pneumonia and infective endocarditis. We propose to investigate the mechanisms of ppGpp and c-di-AMP-regulated resistance to beta-lactam antibiotics and the therapeutic potential of targeting these signalling systems using an in vivo infection model. Reducing MRSA beta-lactam resistance levels would facilitate the use of these antibiotics as monotherapy or in combination with other antimicrobial drugs such as tetracyclines, lincosamides and rifampicin to treat staphylococcal infections previously considered resistant to these agents.
